nix shell: Handle output paths that are symlinks

This requires moving resolveSymlinks() into SourceAccessor. Also, it
requires LocalStoreAccessor::maybeLstat() to work on parents of the
store (to avoid an error like "/nix is not in the store").

Fixes #10375.

src/libutil/source-accessor.hh

@@ -9,6 +9,26 @@ namespace nix {
 
 struct Sink;
 
+/**
+ * Note there is a decent chance this type soon goes away because the problem is solved another way.
+ * See the discussion in https://github.com/NixOS/nix/pull/9985.
+ */
+enum class SymlinkResolution {
+    /**
+     * Resolve symlinks in the ancestors only.
+     *
+     * Only the last component of the result is possibly a symlink.
+     */
+    Ancestors,
+
+    /**
+     * Resolve symlinks fully, realpath(3)-style.
+     *
+     * No component of the result will be a symlink.
+     */
+    Full,
+};
+
 /**
  * A read-only filesystem abstraction. This is used by the Nix
  * evaluator and elsewhere for accessing sources in various
@@ -112,9 +132,9 @@ struct SourceAccessor
         PathFilter & filter = defaultPathFilter);
 
     Hash hashPath(
-            const CanonPath & path,
-            PathFilter & filter = defaultPathFilter,
-            HashAlgorithm ha = HashAlgorithm::SHA256);
+        const CanonPath & path,
+        PathFilter & filter = defaultPathFilter,
+        HashAlgorithm ha = HashAlgorithm::SHA256);
 
     /**
      * Return a corresponding path in the root filesystem, if
@@ -137,6 +157,17 @@ struct SourceAccessor
     void setPathDisplay(std::string displayPrefix, std::string displaySuffix = "");
 
     virtual std::string showPath(const CanonPath & path);
+
+    /**
+     * Resolve any symlinks in `path` according to the given
+     * resolution mode.
+     *
+     * @param mode might only be a temporary solution for this.
+     * See the discussion in https://github.com/NixOS/nix/pull/9985.
+     */
+    CanonPath resolveSymlinks(
+        const CanonPath & path,
+        SymlinkResolution mode = SymlinkResolution::Full);
 };
 
 }